Land-use change: effects on soil carbon, nitrogen and phosphorus pools and fluxes in three adjacent ecosystems

Changes in land use can affect soil organic matter contents and fertility and also atmospheric CO2 concentrations and global warming through soil respiration. We compared total and microbial C, N and P pools and C and N metabolism in sandy loam soils (Typic Udivitrands) under indigenous broadleaf-podocarp forest, grazed introduced pasture and 19-yr old Pinus radiata D. Don forest (planted on previous pasture) in New Zealand. Total and microbial C and N declined consistently with profile depth (except for total N in L and FH samples), and in comparable depths of mineral soil (to 20 cm) tended to be lower in the pine than in the other systems. Total P, organic P and extractable inorganic P concentrations at comparable depths were, in contrast, lowest in the indigenous forest. Microbial P concentrations did not differ significantly between the different systems. Microbial C-to-microbial N ratios differed little among soil profile depths and ecosystems. In 0-10 cm depth mineral soil, CO2-C production, metabolic quotients (qCO2 values) and net N mineralization were all highest in the pasture samples. Net nitrification was high in the pine and pasture samples, but much lower in the indigenous forest samples; nitrate-N was, however, consistently present in streamwater from all three ecosystems. Changes in total C and microbial C and N pools on an area basis to 20 cm depth mineral soil were greatest after conversion of the indigenous forest to pasture; total N contents were, however, as high in the pasture as in the forest and net N mineralization was highest in the pasture. On this area basis, changes in total C contents were small after conversion of pasture to pines, although the distribution within the soil profile did differ considerably between the pine and pasture systems.